The instant invention relates to test fixtures for testing printed circuit boards and more particularly to a head assembly for a vacuum actuated test fixture for testing printed circuit boards.
Vacuum actuated test fixtures have been found to be highly effective for testing many types of printed circuit boards. In this regard, vacuum actuated test fixtures are normally adapted for receiving printed circuit boards of predetermined configurations thereon, and for electrically connecting the printed circuit boards to test computers in order to electrically test the printed circuit boards prior to use. Most of the heretofore available test fixtures of this type have comprised a head assembly which includes an upper peripheral frame, a top plate received the frame, and a flexible diaphragm which extends in sealed relation between the top plate and the frame around the entire perimeter of the frame, so that the top plate is downwardly movable in sealed relation to the frame. A head assembly of this type is normally mounted in a test fixture, so that it is supported above a probe plate in the fixture having a plurality of spring-loaded electrical probes thereon. Accordingly, by evacuating the area between the top plate and the probe plate the top plate can be drawn downwardly toward the probe plate to electrically connect a printed circuit board received on the top plate with the probes on the probe plate. However, it has been found that in many instances the diaphragm of a head assembly of this type can become cracked and worn, and develop vacuum leaks after repeated use. Further, it has been found that the mounting structure, including the peripheral frame required for mounting a diaphragm and a top plate in a head assembly of this type can be relatively large and cumbersome. Accordingly, there is a recognized need for an improved head assembly for a test fixture which enables a top plate to be repeatedly drawn downwardly without eventually effecting the ability of the head assembly to maintain a vacuum in the area between the head assembly and the probe plate. It has also been found that there is a need for a head assembly of this type which can be constructed with a minimum of mounting and supporting structure for achieving a vacuum tight seal around the perimeter of a top plate.
The instant invention provides an improved head assembly for a vacuum actuated test fixture which enables a top plate to be more easily, effectively, and efficiently supported in a manner wherein the top plate is repeatedly movable toward a probe plate over a prolonged period of time without effecting the ability of the head assembly to maintain and hold a vacuum in the area between the top plate and the probe plate. Specifically, the head assembly of the instant invention comprises a top plate, a probe plate having a plurality of electrical probes thereon, and a support assembly for supporting the top plate in upwardly spaced relation to the probe plate and for sealing therebetween in a manner which nevertheless permits the top plate to be moved downwardly relative to the probe plate in order to effect electrical contact between the probes on the probe plate and the printed circuit board. The support assembly comprises a rigid peripheral support member which includes a substantially horizontally extending support wall which extends adjacent to the perimeter of the top plate in downwardly spaced relation thereto, and which is received in sealed relation on the probe plate. The support assembly further comprises a seal assembly between the top plate and the support wall for effecting an airtight seal therebetween. The seal assembly includes a primary seal member which is preferably made of a non-foam elastomer and received in sealed engagement with one of either the bottom side of the top plate or the top side of the support wall, and a resilient secondary seal member which is received in sealed engagement with the other of the bottom side of the top plate or the top side of the support wall. The primary seal member preferably has a channel formed therein and the secondary seal member is preferably received in the channel in the primary seal member so that the primary and secondary seal members cooperate for supporting the top plate on the support wall. The primary seal member is constructed so that it is normally maintained in spaced relation to the other of the bottom side of the top plate or the top side of the support wall by the secondary seal member. However, the secondary seal member is constructed so that it is resiliently compressible in order to draw the primary seal member into sealed engagement with both the bottom side of the top plate and the top side of the support wall. The secondary seal member is preferably received in engagement on top side of the support wall and the primary seal member is preferably supported on the secondary seal member so that it is normally maintained in spaced relation to the top side of the support wall. The primary seal member preferably comprises a non-foam elastomer, and the secondary seal member preferably comprises a resilient foam, such as polyurethane foam. The primary seal member preferably includes inner and outer portions which are disposed on opposite sides of the channel therein, and the inner and outer portions preferably both sealingly engage the other of the bottom side of the top plate or the top side of the support wall when the secondary seal member is compressed to draw the primary seal member into sealed engagement with both the bottom side of the top plate and the top side of the support wall. The support member preferably further includes an outer wall which extends upwardly from the support wall around the perimeter of the top plate and around the seal assembly. The support member preferably still further comprises an inner wall which extends upwardly from the support wall in inwardly spaced relation to the outer wall around the perimeter of the probe plate, and the inner and outer walls of the support member preferably cooperate with the support wall to define an upwardly opening channel for containing the seal assembly therein. Still further, the support member preferably includes an inner lip which extends inwardly from the inner wall and which is received in sealed engagement on the probe plate for securing the support member thereto. The seal member preferably further comprises a spacer portion which extends upwardly between the outer wall and the perimeter of the top plate in order to maintain the perimeter of the top plate in spaced relation to the outer wall.
It has been found that the head assembly of the instant invention can be effectively utilized for providing a compressible seal between the top plate and the probe plate of a printed circuit board test fixture. Specifically, it has been found that because the primary seal member is maintained in sealed engagement with one of either the top plate or the probe plate, and the secondary seal member is maintained in sealed engagement with the other of either the top plate or the probe plate, the seal assembly is operative for providing an effective seal between the top plate and the probe plate regardless of whether the secondary seal member is in a compressed or an uncompressed disposition. Further, when a vacuum is applied to the area between the top plate and the probe plate, the primary seal member is drawn into pressurized sealed engagement with both the top plate and the support wall to provide an even more effective airtight seal between the top plate and the probe plate. Still further, because the head assembly of the subject invention relies on the first and second seal members to effect a seal between the top plate and the probe plate rather than utilizing a flexible diaphragm, the head assembly of the subject invention is substantially more durable than the previously available seal assemblies. Even still further, because of the construction of the seal assembly of the instant invention the primary and secondary seal members can be readily and easily replaced if either is damaged.
Accordingly, it is a primary object of the instant invention to provide a head assembly for a printed circuit board test fixture which can withstand repeated use for a prolonged period of time.
Another object of the instant invention is to provide a head assembly for a printed circuit board test fixture which includes a readily replaceable and removable seal assembly.
An even still further object of the instant invention is to provide a seal assembly for a printed circuit board test fixture which includes a support member formed in the configuration of an upwardly-opening channel, and a seal assembly in the channel comprising an elastomeric primary seal member and a resilient foam secondary seal member, wherein the secondary seal member is resiliently compressible to enable the primary seal member to be drawn into pressurized sealed engagement with both the support member and the top plate of the test fixture.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.